Problem: Which of these numbers is composite? ${2,\ 13,\ 43,\ 55,\ 79}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 2, 13, 43, and 79 each have only two factors. The factors of 55 are 1, 5, 11, and 55. Thus, 55 is the composite number.